Austenitic steel is used in various applications owing to characteristics thereof such as work hardenability and non-magnetic properties. Particularly, although ferritic or martensitic carbon steel having ferrite or martensite as a main microstructure thereof has been widely used, the characteristics of ferritic or martensitic carbon steels are limited, and thus the use of austenitic steel has increased as a substitute therefor, overcoming the disadvantages of ferritic and martensitic steels.
The use of austenitic steel has steadily increased in many industrial applications requiring steel having ductility and resistance to wear and hydrogen embrittlement, such as in rails for maglev rail systems; nonmagnetic structural members for general electrical devices and superconducting devices of nuclear fusion reactors; mining machinery in mines; general transportation; pipe expanding devices; slurry pipes; anti souring gas materials; and materials for mining, transportation, and storage in the oil and gas (petroleum) industries.
In the related art, austenitic stainless steel AISI304 (18Cr-8Ni) is a typical nonmagnetic steel material. However, such austenitic stainless steel is not suitable for structural members due to having low yield strength, and is not economical because large amounts of relatively expensive chromium (Cr) and nickel (Ni) are included. Particularly, since austenitic stainless steel is converted into a magnetic material if ferrite having ferromagnetic characteristics is formed therein by strain induced transformation, the austenitic stainless steel is not suitable for structural members requiring stable nonmagnetic characteristics not varying according to load. That is, the applications of austenitic stainless steel are limited.
Furthermore, along with the development, of the mining, oil, and gas industries, the wear on steel used for mining, transportation, and refining applications has become problematic. Particularly, although oil sands have been recently developed in earnest as an unconventional source of petroleum, the wear on steel members caused by slurry containing oil, gravel, and sand is one of the main factors increasing the production cost of oil from oil sands, and thus, the development and practical implementation of steel having a high degree of resistance to wear are increasingly required. In the mining industry, Hadfield steel having high wear resistance has commonly been used. Hadfield steel is austenitic steel in which the transformation of a microstructure to martensite having a high degree of hardness takes place in response to deformation.
The microstructure of such varied kinds of austenitic steel may be maintained as austenite by increasing the contents of manganese and carbon therein. In this case, however, carbides may be formed at high temperature along grain boundaries of austenite in the form of a network, thereby worsening characteristics of the austenitic steel, particularly, ductility of the austenitic steel. In addition thereto, larger amounts of carbides are formed in welded portions (weld heat affected zones) which are heated to high temperatures and subsequently cooled, and thus the toughness of the weld heat affected zones is markedly decreased.
A method of manufacturing high-manganese steel by rapidly cooling high-manganese steel to room temperature after a solution heat treatment or a hot working process, performed on high-manganese steel at a high temperature, has been proposed to prevent the formation of network-shaped carbide precipitates. However, if a thick steel sheet is formed by the proposed method, the effect of preventing the precipitation of carbides is not sufficiently obtained by rapid cooling. In addition, the precipitation of carbides may not be prevented in weld heat affected zones due to the effect of the heat history of the weld heat affected zones.
Furthermore, since the machinability of austenitic high-manganese steel is worsened due to a high degree of work hardenability, the lifespans of cutting tools may be decreased, and thus, costs for cutting tools may be increased. In addition, process suspension times may be increased due to the need for the frequent replacement of cutting tools. Thus, manufacturing costs may be increased.